Electrophotography



United States Patent ELECTROPHOTOGRAPHY John J. Rheinfrank, Worthington,and James M. Chapman, Dayton, Ohio, asslgnors, by mesne assignments, toThe Haloid Company, Rochester, N. Y., a corpora tion of New York NoDrawing. Application April 10, 1953, Serial No. 348,126

5 Claims. (Cl. 95-13) This invention relates to electrophotography orxerography, and has for its object to improve the xerographic processesas heretofore known or practiced.

A particular purpose of the invention is to improve the functioning andefliciency of the electrophotographic or xerographic plate, particularlywhen used for repeated operation.

To these and other ends, the invention consists in the procedure thatwill appear clearly from the following description, the novel featuresbeing pointed out in the claims following the specification.

This application is a continuation-in-part of Serial No. 83,104, filedMarch 23, 1949, now abondoned.

In the electrophotographic or xerographic process, as described inCarlson Patent No. 2,297,691, October 6, 1942, for example, anelectrophotographic plate or xeroplate, comprising a coating ofphotoconductive insulating material, such as selenium, on a conductivebacking, is electrically charged so that the coating carries a fairlyuniform positive or negative charge over its surface. The plate is thenexposed to a light image to discharge the surface in accordance with thelight falling on it and leave an electrostatic latent image. The imageis then developed by depositing a finely divided, electrostaticallyattractable material, such as powder, thereon. Finally, the material orpowder is transferred to and aflixed on another surface, such as a sheetof paper, to produce a print. The plate is thereupon cleaned of residualpowder and recharged in preparation for another exposure to the same ordifierent subject matter.

The charging of the plate may be accomplished by frictional rubbing asdescribed in the Carlson patent or by other methods, such as by passingthe plate under a corona discharge emanating from a row of needles or agrid of fine wires as described, for example, in copending application,Serial No. 55,526, filed October 20, 1948, now abandoned.

It has been noticed that if a plate is used repeatedly for reproducingseveral images in a short period of time by charging the plate, exposingto a light image, and cascading developing powder, such as a finelydivided, pigmented resin over its surface, then transferring or removing the powder image and repeating the cycle of charging the plate,exposing to the same or a different light image and developing withpowder, and so forth through several more cycles of operation, thesuccessive powder images obtained on the plate in successive cyclesoften exhibit a pronounced decrease in density and legibility.

This decrease in image density and legibility can be attributed to adecrease in the potential acquired by the plate during charging. TableI, for example, illustrates the decay in potential acquired by aselenium-coated plate during successive charging cycles. In each cycle,the plate coating was subjected to a positive corona discharge bypassing it under a row of needles charged to 5000 volts and thepotential of the plate surface was then measured with an electrometerand probe. The plate 2,741,959 Patented Apr. 17, 1956 was thendischarged by exposure to light and the charging cycle repeated.

Table I Measured Voltage on Char i Cycle 00mm Pcfi fial Needles (Volts)It will be noted that the potential acquired by the plate graduallydecreased with successive charging cycles. This etfect is typical of thephenomenon known as plate fatigue, and is a serious problem incontinuous or repetitive electrophotographic processes. The presentinvention relates specifically to the regeneration of fatigued plates.

it has now been discovered that plates can be regenerated by introducingan additional charging step between successive copying cycles. Thisadditional charging step imposes upon the plate a charge of polarityopposite to that of the charge used in sensitizing the plate. Thus, ifthe plate has been used with positive sensitization for the copyingcycle, it has been found that the plate becomes regenerated when anegative charge is imposed upon the plate prior to the next copyingcycle. Such a negative charge may be imposed by passing the plate undercorona discharge wires or needles which are charged negatively. Uponbeing resensitized by positive corona (as the charging step in the nextcopying cycle), the plate will be found to have regained its originalability to acquire a substantial initial potential and to produce imageshaving good density and legibility.

Fatigue occurs when the combined operations of charging and exposing anelectrophotographic plate are repeated. The present regeneration processcomprises charging the plate to a polarity which is the reverse of thepolarity of the original sensitizing charge, before repeating the stepsof sensitizing (by charging) and exposing the plate. This regenerationstep may be performed at any time between the development step or otherutilization of the electrostatic image of one copying cycle and thesensitizing step of the succeeding copying cycle. Thus, for example, theregeneration step may be performed either just before or just after thetransfer step, or just following the cleaning step in the copying cycle.

The regeneration step may be employed only when the effects of fatiguebecome noticeable, rather than between each successive copying cycle, ifthe operator so desires.

Electrometer tests indicate that regenerated plates will acquiresubstantially their original potential on resensitization. Table II,which is a continuation of Table I, shows the effect on the potentialacquired by a positively sensitized plate, when regeneration is effectedby imposing a negative corona discharge between successive copyingcycles.

Table II Measured Plate Potential (Volts) Voltage on Corona NeedlesCharging Cycle While the present invention, as to objects andadvantages, has been described herein as carried out in a specificembodiment thereof, it is not desired to be limited thereby but it isintended to cover the invention broadly within the spirit and scope ofthe appended claims.

7 What is claimed is:

1. Ina repetitive electrophotographic process which comprises the stepsof sensitizing a photoconducting insulating layer on a conductingbacking by charging it with an electrostatic charge of a first polarity,exposing said coating to a light image, developing an image on saidcoating by depositing a finely divided material thereon, transferringthe image so developed, cleaning said coating and repeating the abovecycle of operations, the added steps comprising exposing said coating toa source of electrical charge and energizing said source with apotential opposite in polarity to said first polarity, said added stepsbeing performed subsequent to the developin step and prior to the stepof sensitizing the coating in repeating the cycle.

2. In 'a repetitive electrophotographic process which comprises thesteps of sensitizing a selenium coating on a conductive backing byelectrically charging said coating to a positive polarity, exposing thesensitized coating to a light image, developing an image on said coatingwith a finely divided material, transferring said image, cleaning saidcoating, and repeating the above cycle of operations, the added stepscomprising exposing said selenium coating to a source of electricalcharge and energizing said source to impose an electrostatic charge ofnegative polarity on said coating, said added steps being performedafter the developing step and prior to recharging the selenium coatingto a positive polarity in repeating the process.

3. In the method of electrophotography in which a coating ofphotoconductive insulating material on a conductive backing iselectrically charged to afirst polarity, exposed to a light image,developed with a finely divided material and then relieved of saiddeveloping material by transfer and cleaning, after which said cycle ofoperations is repeated, the added steps comprising exposing said d 11coating to a source of electrical charge and energizing said source witha potential opposite in polarity to said first potential, said addedsteps being performed after cleaning the coating and before rechargingto said first polarity in repeating said cycle.

4. The method of copying light images which comprises charging a coatingof photoconductive insulating material on a conductive backing with anelectrostatic charge of a first polarity, exposing said coating toalight image, developing said coating by depositing a finely dividedmaterial thereon. transferring said developed image, cleaning saidcoating, charging said coating with an electrostatic charge opposite insign to said first polarity, then recharging said coating with anelectrostatic charge of said first polarity prior to a second exposureto a light image.

5. In a repetitive electrophotographic process which comprises the stepsof sensitizing a photoconducting insulating layer on a conductingbacking bycharging it with an electrostatic charge of a first polarity,exposing said coating to a light image to provide a latent electrostaticimage, utilizing said electrostatic imagein a copying cycle to produce apowder image corresponding thereto, exposing to relatively uniform lightbetween copying cycles,

and repeating the entire series of operations, the'added stepscomprising exposing the coating to a source Of electrical charge andenergizing said source with a potential opposite in polarity to saidfirst polarity, said added steps being performed subsequent to theutilization of the electrostatic image and prior to the step ofsensitizing the coating in repeating the cycle.

2,551,582 Carlson May 8, 1951 Schaffert Nov. 20, 1'95l

1. IN A REPETITIVE ELECTROPHOTOGRAPHIC PROCESS WHICH COMPRISES THE STEPSOF SENSITIZING A PHOTOCONDUCTING INSULATING LAYER ON A CONDUCTINGBACKING BY CHARGING IT WITH AN ELECTROSTATIC CHARGE OF A FIRST POLARITY,EXPOSING SAID COATING TO A LIGHT IMAGE, DEVELOPING AN IMAGE ON SAIDCOATING BY DEPOSITING A FINELY DIVIDED MATERIAL THEREON, TRANSFERRINGTHE IMAGE SO DEVELOPED, CLEANING SAID COATING AND REPEATING THE ABOVECYCLE OF OPERATIONS, THE ADDED STEPS COMPRISING EXPOSING SAID COATING TOA SOURCE OF ELECTRICAL CHARGE AND ENERGIZING SAID SOURCE WITH APOTENTIAL OPPOSITE IN POLARITY TO SAID FIST POLARITY, SAID ADDED STEPSBEING PERFORMED SUBSEQUENT TO THE DEVELOPING STEP AND PRIOR TO THE STEPOF SENSITIZING THE COATING IN REPEATING THE CYCLE.